Abstract

Actively granting food to a companion is called pro-social behavior and is considered to be part of altruism. Recent findings show that some non-human primates behave pro-socially. However, pro-social behavior is not expected in despotic species, since the steep dominance hierarchy will hamper pro-sociality. We show that some despotic long-tailed macaques do grant others access to food. Moreover, their dominance hierarchy determines pro-social behavior in an unexpected way: high-ranking individuals grant, while low-ranking individuals withhold their partner access to food. Surprisingly, pro-social behavior is not used by subordinates to obtain benefits from dominants, but by dominants to emphasize their dominance position. Hence, Machiavellian macaques rule not through "fear above love", but through "be feared when needed and loved when possible".

The drawing shows the subject in the middle compartment having the choice between either granting itself and its partner (in compartment three) access to a banana (choice B, the ‘pro-social’ choice), or granting only itself access to a banana and leaving a banana in front of an empty compartment (compartment one) (choice A, the ‘a-social’ choice).

a. Pro-sociality and kin. Mean preference ± s.e.m. for the partner-side of all individuals tested with their kin (n = 10) in the test condition, the mean preference ± s.e.m. for the same side of the same individuals in the control condition and the mean preference ± s.e.m. for the partner-side of the same individuals when tested with non-kin. b. Pro-sociality and non-kin. Mean preference ± s.e.m. for the partner-side of all individuals tested with non-kin (n = 20) in the test condition and the mean preference ± s.e.m. for the same side of the same individuals in the control condition * indicates a difference at the p<0.05 level (exact Wilcoxon signed ranks test).

Pro-social tendency (difference between the preference for partner side in the test condition and the preference for the same side in the control condition) and absolute rank number (nr 1 is the alpha male) of all subjects towards kin (open circles and dotted line) and non-kin (closed circles and full line). Lines indicate linear regressions significant at the p<0.05 level.